Eliminate FontBuilder::initForStyleResolve.

It's not necessary to do initialization of FontBuilder separately from
StyleResolverState::setStyle. It just adds noise.

Also, use a reference for Document in FontBuilder, since its existence can
be guaranteed in all cases.

R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/671613003

git-svn-id: svn://svn.chromium.org/blink/trunk@184342 bbb929c8-8fbe-4397-9dbb-9b2b20218538